The process of soldering is one of the most widely used joining techniques. Even wider use of soldering for joining workpieces would be possible absent several limitations inherent in conventional solder processes. Typically, conventional solder processes can be used successfully only (1) if the surfaces of the workpieces to be joined are cleaned and free of any oxide, nitride, or other stable compound layers present before application of the solder, to ensure good contact between the solder and the workpiece surfaces; (2) if a precoating with a flux is used at the same time as the solder; or (3) if the workpiece surfaces are cleaned and a precoating with a flux is used. These limitations mean that the workpiece surfaces to be soldered require a complicated pretreatment, that the soldering operation is made more complex by the use of added flux, or both. In addition, the risk exists that, after the soldering process, flux residues will remain on the soldered workpiece surfaces. Residual flux may cause problems in further processing steps or impair the long-term durability of the soldered joints.
Other conventional solder processes involve plating with a metal (e.g., Au) that does not form a stable oxide or compound upon heating or with a metal (e.g., Cu or Ni) that forms an oxide that is easily removed by fluxes. These processes require that the workpiece surfaces to be soldered receive multi-step pretreatment or plating, or that the soldering operation involves a multi-step procedure including the added use of plating, flux, or both. Some of these conventional soldering processes are hazardous to health, the environment, or both.
Other commercial soldering processes use soft solder alloys, which comprise tin and/or lead and possibly silver, and have a process temperature of about 180-280° C. Such processes have the further limitation that they wet many materials either not at all or only very poorly. Therefore, these processes cannot be used to join workpieces made of poorly wettable or entirely nonwettable materials such as ceramics.
Some soldering processes attempt to overcome this limitation by using activated soft solders. The activated soft solders, with a proportion of titanium as the “active” metal, exhibit significantly improved wetting even of surfaces that are poorly wettable. A significant limitation of the processes using active solders is, however, that they require process temperatures between 600° C. to 900° C. and require a protective atmosphere such as a high vacuum or a pure shielding gas. The high processing temperature severely limits the choice of solderable materials. Furthermore, the need for a vacuum or shielding gas complicates the soldering operation and, in many cases, precludes application of the process at all.
The active solders that have been recently developed include a certain amount of reactive elements, rare earth or lanthanide series elements (La, Ce, Ga, etc.), in combination with the titanium (or other elements of subgroup IVa and/or Va of the Periodic Table of the Elements) and conventional solder metals (i.e., tin, silver, indium, and the like). Such solder alloys wet and bond to a wider variety of materials than the unmodified solders would normally. These compositions are generally used to bond dissimilar materials, such as aluminum to copper or titanium to silicon carbide, or to join difficult-to-bond materials such as tungsten or titanium. These materials can also be useful in bonding silicon to electronic packaging materials such as alumina, copper-tungsten alloy, copper-molybdenum-copper alloy, and metal-ceramic composites such as aluminum silicon carbide.
The recently developed active solders have melting points above 200° C. (at 233° C. for a typical Sn—Ti—Ag—Ce—Ga alloy) and render joining successful at temperatures from 250-400° C. Solder processes that use these active solders are limited, however, to joining applications where joining temperatures are permitted to be above 250° C. There are many emerging applications in microelectronics; sensors; micro-electro-mechanical (MEM) devices; and optical, polymer-coated optical, or metal-coated polymer devices that require solders that join below 180° C.
Conventional packaging of semiconductor devices uses a filled organic polymer, filled silicone polymer, or filled grease material to create a thermal pathway between the device and the package lid. These materials typically have no capability to join or bond, but must stay in place through normal use of the device. Replacement of these organic materials with metals, such as solders, substantially improves the rate of heat transfer out of the device, and can also aid in attaching the packaging materials to the silicon. The approximately ten-fold increase in thermal conductivity found with metals allows devices to perform better by allowing operation at higher speeds, allowing an increased transistor count, or allowing a less complex system to remove the heat generated.
As mentioned above, however, conventional solder materials bond to semiconductor materials, ceramics, and metals only if the surface is pretreated to minimize the oxide naturally present on the surface of the substrate materials. The most common treatment is the use of a fluxing agent, which is a chemical designed to reduce the oxides present on the surfaces while not interfering with the attachment of the solder to the substrate. In semiconductor devices, where contamination can be catastrophic to their operation, the use of fluxes is not desirable. Instead, a layer of gold is frequently added to the surface of the device as well as the packaging material. This is done by one of several conventional processes, such as sputtering or chemical vapor deposition.
Because gold can diffuse into materials, and possibly damage a semiconductor device, or may have difficulty firmly attaching to some materials, adhesion and barrier layers are also added between the gold coating and the substrate surface. A common combination is nickel as an adhesion layer directly on the device or substrate, titanium or palladium as a barrier layer to prevent gold from diffusing into the substrate, and finally the gold layer. If the gold layer is processed correctly and kept free of excess oxide formation, then some conventional solders will bond directly to the gold without the use of flux.
Unfortunately, the creation of the additional layers adds multiple steps to the production process, increasing cost, increasing the risk of part failure during processing, and possibly not improving the thermal performance of the total package due to the resulting relatively thick metal layer between the device and its package. For certain special cases, the process of eutectic copper bonding can be used; this process is even more expensive and complicated.
In view of the shortcomings of existing active solder alloy compositions and processing methods, a need remains for a new alloy composition and processing method for making higher-strength joints. This need is especially acute in the areas of electronic packaging, optical packaging, cold plates, and heat spreaders.
Therefore, an object of the present invention is to provide a soldering process, for joining workpieces, which enables more versatile applicability of the soft-solder technique. More specific objects of the present invention are to provide a soldering process that functions not only in inert and other protective atmospheres but even in oxygen-containing atmospheres such as, for example, in air; has a relatively low processing temperature; and wets even poorly wettable surfaces. Still another object of the present invention is to provide a process that avoids the need for a flux.
In particular, it is an object of the invention to provide a low-temperature (less than about 180° C.) active solder alloy. A related object is to introduce a method to join ceramic, silicon, composite, or corrosion-resistant metals, using such low-temperature active solders. Another related object is to identify applications where such a low-temperature active solder would be used. Like other active solders, the compositions of the present invention can be processed in air and will wet surfaces that per se are poorly wettable, such as ceramics, silicon, composites, or metals with ceramic layer surfaces.